1 Introduction

Gravitational waves, one of the more exotic predictions of Einstein’s General Theory of Relativity may,
after decades of controversy over their existence, be detected within the next ten years.

Sources such as interacting black holes, coalescing compact binary systems, stellar collapses and pulsars
are all possible candidates for detection; observing signals from them will significantly boost our
understanding of the Universe. New unexpected sources will almost certainly be found and time will tell
what new information such discoveries will bring. Gravitational waves are ripples in the curvature of
space-time and manifest themselves as fluctuating tidal forces on masses in the path of the wave. The first
gravitational wave detectors were based on the effect of these forces on the fundamental resonant mode of
aluminium bars at room temperature. Initial instruments were constructed by Joseph Weber [104, 105] and
subsequently developed by others. Reviews of this early work are given in [101, 22]. Following
the lack of confirmed detection of signals, aluminium bar systems operated at and below the
temperature of liquid helium were developed and work in this area is still underway [70, 73, 2, 41].
However the most promising design of gravitational wave detectors, offering the possibility of
very high sensitivities over a wide range of frequency, uses widely separated test masses freely
suspended as pendulums on earth or in a drag free craft in space; laser interferometry provides
a means of sensing the motion of the masses produced as they interact with a gravitational
wave.

Ground based detectors of this type, based on the pioneering work of Bob Forward and
colleagues (Hughes Aircraft) [67], Rai Weiss and colleagues (MIT) [107], Ron Drever and colleagues
(Glasgow/Caltech) [24, 23] and Heinz Billing and colleagues (MPQ Garching) [6], will be used to observe
sources whose radiation is emitted at frequencies above a few Hz, and space borne detectors, as originally
envisaged by Peter Bender and Jim Faller [20, 28] at JILA will be developed for implementation at lower
frequencies.

Already gravitational wave detectors of long baseline are being built in a number of places around the
world; in the U.S.A. (LIGO project led by a Caltech/MIT consortium) [4, 56], in Italy (VIRGO project, a
joint Italian/French venture) [13, 102], in Germany (GEO 600 project being built by a collaboration
centred on the University of Glasgow, the University of Hannover, the Max Planck Institute for Quantum
Optics, the Max Planck Institute for Gravitational Physics (Albert Einstein Institute), Golm and the
University of Wales, Cardiff) [45, 33] and in Japan (TAMA 300 project) [99, 95]. A space-borne detector,
LISA [19, 58, 57], – proposed by a collaboration of European and U.S. research groups – has been adopted
by ESA as a future Cornerstone Mission. When completed, this detector array should have the
capability of detecting gravitational wave signals from violent astrophysical events in the Universe,
providing unique information on testing aspects of General Relativity and opening up a new field of
astronomy.

We recommend a number of excellent books for reference. For a popular account of the development of
the gravitational wave field the reader should consult Chapter 10 of ‘Black Holes and Time Warps’ by Kip
S. Thorne [97]. A comprehensive review of developments toward laser interferometer detectors is found in
‘Fundamentals of Interferometric Gravitational Wave Detectors’ by Peter Saulson [86], and discussions
relevant to the technology of both bar and interferometric detectors are found in ‘The Detection of
Gravitational Waves’ edited by David Blair [7]. In addition to the home site of this journal and the sites
listed above there is a very informative general site maintained by the National Centre for Supercomputing
Applications [74].